Microelectronic elements such as semiconductor chips ordinarily are provided in packages which physically and chemically protect the microelectronic element itself, and which facilitate mounting and electrical connection of the microelectronic element to a larger circuit. For example, semiconductor chips typically are flat bodies having generally planar front and rear surfaces, with contacts disposed on the front surface connected to the internal electrical circuitry of the chip itself. Semiconductor chips typically are provided in packages which enclose the chip and which define terminals electrically connected to the contacts of the chip itself. By placing the package on a circuit panel and connecting the terminals to the circuit panel, the chip can be mounted and connected to the circuit.
One type of microelectronic package is commonly referred to as a “lead frame” package. A lead frame package begins with a self-supporting metallic element referred to as the “lead frame” incorporating terminals and strips of relatively thick metal connecting the terminals to bus bars formed integrally with the strips and terminals. Such a lead frame may be fabricated by conventional metal working processes using dies to punch out unwanted areas from a metal sheet, or by etching a metal sheet. The lead frame is assembled with a microelectronic element such as a semiconductor chip, and the contacts of the chip are connected to individual metallic strips so that the metallic strips serve as leads connecting the contacts of the chip to the terminals. The resulting assembly is then encapsulated, typically by applying a protective material referred to as an “overmold,” leaving the bus bars and portions of the strips adjacent the bus bars projecting from the overmolding. The bus bars are then removed. In some lead frame packages, relatively large portions of the strips project from the overmold, and the bus bar removal process is conducted so as to leave these large portions projecting substantial distances from the overmold. The ends of the strips remote from the overmold constitute the terminals. In one form of lead frame package, commonly referred to as a Thin Small-Outline Package or “TSOP”, the projecting lead portions are bent downwardly, so that the ends of these portions forming the terminals lie in a plane below the overmold. In a variant of this process, the projecting portions of the strips may be bent inwardly so that they extend underneath the bottom surface of the overmold. Lead frame packages such as TSOPs are economical to manufacture, and to mount on a circuit board.
An important consideration in forming a lead frame package is the input/output impedance of the terminals. In order to minimize signal reflection when a signal is transmitted to a terminal, the input impedance of the terminal should be matched to the impedance of the source supplying the signal. Similarly, when a signal is transmitted from a terminal, the output impedance of the terminal should be matched to the input impedance of the load to which the signal is supplied. To provide such matching, the characteristic impedance of the conductors extending between the chip and the terminals of the package can be controlled to a preselected value, most commonly 50 or 75 ohms. However, lead frame packages such as TSOPs typically do not provide such impedance control. As chip operating speeds increase, the need for impedance control becomes more acute.